Evolution of complex adaptation requires a match between the functional relationships of the phenotypic characters and their genetic representation. This was clearly expressed by Rupert Riedl (1975) in his thesis of the "imitatory epigenotype." If the epigenetic regulation of gene expression "imitates" the functional organization of the traits then the improvement by mutation and selection is facilitated. Riedl predicts that the evolution of the genetic representation of phenotypic characters tends to favor those representations which imitate the functional organization of the characters. Imitation means that complexes of functionally related characters shall be "coded" as developmentally integrated characters but coded independently of functionally distinct character complexes.
Independent genetic representation of functionally distinct character complexes can be described as modularity of the genotype-phenotype mapping functions. A modular representation of two character complexes C1 and C2 is given if pleiotropic effects of the genes are more frequent among the members of a character complex than among members of different complexes (see Fig. 1).
Figure:
Example of a modular representation of the character complexes C1={A, B, C, D}
and C2={E, F, G} which serve to functions F1 and F2.
Each character complex has a primary function, F1 for C1 and F2 for C2.
Only weak influences exist of C1 on F2 and vice versa. The genetic
representation is modular because the pleiotropic effects of the
genes M1={G1, G2, G3} have primarily pleiotropic effects on the
characters in C1 and M2={G4, G5, G6} on the characters in complex C2.
There are more pleiotropic effects on the characters within each complex
than between them.
The concept of modularity was clearly expressed by John Bonner in his concept of gene nets (Bonner, 1988):
"I will call [...] a 'gene net' [...] a grouping of a network of gene actions and their products into discrete units during the course of development." (P174) "This general principle of the grouping of gene products and their subsequent reactions into gene nets becomes increasingly prevalent as organisms become more complex. This not only was helpful and probably necessary for the success of the process of development, but it also means that genetic change can occur in one of these gene nets without influencing the others, thereby much increasing its chance of being viable. The grouping leads to a limiting of pleiotropy and provides a way in which complex developing organisms can change in evolution." (p. 175, emphasis by GPW)
The idea that development is organized into semi-autonomous processes is actually much older, dating back to the beginnings of developmental biology and was summarized under the term "dissociability" by Needham (1933). Needham pointed out that even if development is a perfectly integrated process its component parts can be disentangled experimentally: growth can occur without differentiation and nuclear division without cell division and so on. The evolutionary importance of this fact was emphasized by Gould (1977, p 234) who suggested that dissociability is the developmental prerequisite for heterochronic change (see also Raff and Kaufman, 1983, p 150; Raff, 1995).
The existence of semi-autonomous units of the phenotype might be particularly important in connection with sexual reproduction (Stearns, 1993). Sexual reproduction rearranges genetic variation in every generation which creates the problem of maintaining functional phenotypic units intact. Stabilizing the development of functionally related character complexes allows to recombine integrated traits rather than true "random" variation.
The fact that the morphological phenotype can be decomposed into basic organizational units, the homologues of comparative anatomy, has also been explained in terms of modularity. It has been suggested that properly identified homologues are developmentally and genetically individualized parts of the organisms (Wagner, 1989b,c). The biological significance of these semi-autonomous units is their possible role as adaptive "building blocks" (Wagner, 1995).